unison/unison-src/transcripts/merges.output.md

Forking and merging namespaces in ucm

The Unison namespace is a versioned tree of names that map to Unison definitions. You can change this namespace and fork and merge subtrees of it. Let's start by introducing a few definitions into a new namespace, foo:

x = 42

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      x : Nat

.> add

  ⍟ I've added these definitions:
  
    x : Nat

Let's move x into a new namespace, master:

.> rename.term x master.x

  Done.

If you want to do some experimental work in a namespace without disturbing anyone else, you can fork it (which is a shorthand for copy.namespace). This creates a copy of it, preserving its history.

Note: these copies are very efficient to create as they just have pointers into the same underlying definitions. Create as many as you like.

Let's go ahead and do this:

.> fork master feature1
.> view master.x
.> view feature1.x

Great! We can now do some further work in the feature1 branch, then merge it back into master when we're ready.

y = "hello"

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      y : Text

  ☝️  The namespace .feature1 is empty.

.feature1> add

  ⍟ I've added these definitions:
  
    y : Text

.master> merge .feature1

  Here's what's changed in the current namespace after the
  merge:
  
  Added definitions:
  
    1. y : Text
  
  Tip: You can use `todo` to see if this generated any work to
       do in this namespace and `test` to run the tests. Or you
       can use `undo` or `reflog` to undo the results of this
       merge.

  Applying changes from patch...

.master> view y

  y : Text
  y = "hello"

Note: merge src, with one argument, merges src into the current namespace. You can also do merge src dest to merge into any destination namespace.

Notice that master now has the definition of y we wrote.

We can also delete the fork if we're done with it. (Don't worry, even though the history at that path is now empty, it's still in the history of the parent namespace and can be resurrected at any time.)

.> delete.namespace .feature1

  Done.

.> history .feature1

  ☝️  The namespace .feature1 is empty.

.> history

  Note: The most recent namespace hash is immediately below this
        message.
  
  ⊙ 1. #733ouv89e4
  
    - Deletes:
    
      feature1.y
  
  ⊙ 2. #nomh416gj1
  
    + Adds / updates:
    
      master.y
    
    = Copies:
    
      Original name New name(s)
      feature1.y    master.y
  
  ⊙ 3. #bsnvm0os4j
  
    + Adds / updates:
    
      feature1.y
  
  ⊙ 4. #gipijd9j3c
  
    > Moves:
    
      Original name New name
      x             master.x
  
  ⊙ 5. #2dhojv53dl
  
    + Adds / updates:
    
      x
  
  □ 6. #gndb53fevj (start of history)

To resurrect an old version of a namespace, you can learn its hash via the history command, then use fork #namespacehash .newname.

Concurrent edits and merges

In the above scenario the destination namespace (master) was strictly behind the source namespace, so the merge didn't have anything interesting to do (Git would call this a "fast forward" merge). In other cases, the source and destination namespaces will each have changes the other doesn't know about, and the merge needs to something more interesting. That's okay too, and Unison will merge those results, using a 3-way merge algorithm.

Note: When merging nested namespaces, Unison actually uses a recursive 3-way merge, so it finds a different (and possibly closer) common ancestor at each level of the tree.

Let's see how this works. We are going to create a copy of master, add and delete some definitions in master and in the fork, then merge.

.> fork master feature2

  Done.

Here's one fork, we add z and delete x:

z = 99

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      z : Nat

.feature2> add

  ⍟ I've added these definitions:
  
    z : Nat

.feature2> delete.term.verbose x

  Name changes:
  
    Original         Changes
    1. feature2.x ┐  2. feature2.x (removed)
    3. master.x   ┘  
  
  Tip: You can use `undo` or `reflog` to undo this change.

And here's the other fork, where we update y and add a new definition, frobnicate:

master.y = "updated y"
master.frobnicate n = n + 1

  Loading changes detected in scratch.u.

  I found and typechecked these definitions in scratch.u. If you
  do an `add` or `update`, here's how your codebase would
  change:
  
    ⍟ These new definitions are ok to `add`:
    
      master.frobnicate : Nat -> Nat
      master.y          : Text

.> update

  Okay, I'm searching the branch for code that needs to be
  updated...

  Done.

.> view master.y

  master.y : Text
  master.y = "updated y"

.> view master.frobnicate

  master.frobnicate : Nat -> Nat
  master.frobnicate n =
    use Nat +
    n + 1

At this point, master and feature2 both have some changes the other doesn't know about. Let's merge them.

.> merge feature2 master

  Here's what's changed in master after the merge:
  
  Added definitions:
  
    1. z : Nat
  
  Removed definitions:
  
    2. x : Nat
  
  Tip: You can use `todo` to see if this generated any work to
       do in this namespace and `test` to run the tests. Or you
       can use `undo` or `reflog` to undo the results of this
       merge.

  Applying changes from patch...

Notice that x is deleted in the merged branch (it was deleted in feature2 and untouched by master):

.> view master.x

  ⚠️
  
  The following names were not found in the codebase. Check your spelling.
    master.x

And notice that y has the most recent value, and that z and frobnicate both exist as well:

.> view master.y

  master.y : Text
  master.y = "updated y"

.> view master.z

  master.z : Nat
  master.z = 99

.> view master.frobnicate

  master.frobnicate : Nat -> Nat
  master.frobnicate n =
    use Nat +
    n + 1

FAQ

• What happens if namespace1 deletes a name that namespace2 has updated? A: ???
• ...